Conductivity modulated drain extended MOSFET

ABSTRACT

An integrated circuit is fabricated on a semiconductor substrate. An insulated gate bipolar transistor (IGBT) is formed upon the semiconductor substrate in which the IGBT has an anode terminal, a cathode terminal, and a gate terminal, and a drift region. A diode is also formed on the semiconductor substrate and has an anode terminal and a cathode terminal, in which the anode of the diode is coupled to the anode terminal of the IGBT and the cathode of the diode is coupled to the drift region of the IGBT.

TECHNICAL FIELD OF THE DISCLOSURE

This disclosure relates drain extended MOSFETS, and in particular toinsulated-gate bipolar transistors.

BACKGROUND

An insulated-gate bipolar transistor (IGBT) is a three-terminal powersemiconductor device primarily used as an electronic switch. It combineshigh efficiency and fast switching. IGBTs switch electric power in manymodern appliances: variable-frequency drives (VFDs), electric cars,trains, variable speed refrigerators, lamp ballasts, air-conditioners,and even stereo systems with switching amplifiers, etc. Since it isdesigned to turn on and off rapidly, amplifiers that use it oftensynthesize complex waveforms with pulse-width modulation and low-passfilters. In switching applications, modern devices feature pulserepetition rates well into the ultrasonic range-frequencies which are atleast ten times the highest audio frequency handled by the device whenused as an analog audio amplifier.

The IGBT combines the simple gate-drive characteristics of a metal oxidesemiconductor field effect transistor (MOSFET) with the high-current andlow-saturation-voltage capability of a bipolar transistor. The IGBTcombines an isolated-gate FET for the control input and a bipolar powertransistor as a switch in a single device. Large IGBT modules typicallyinclude many devices in parallel and can have very high current-handlingcapabilities in the order of hundreds of amperes with blocking voltagesof 6000 V. These IGBTs can control loads of hundreds of kilowatts.

Various structures for IGBTs, such as: planar IGBTs, trench IGBTs, andlateral IGBTs, have been designed to customize the operationalproperties of the device for particular applications. For example,planar or vertical IGBTs utilize a convenient structure for a high power(e.g., high voltage and high current) switch. The planar IGBT includes acollector at a bottom side, a gate at a top side, and an emittersurrounding the gate at the top side. Trench gate IGBTs have a similargeneral structure to the planar IGBTs. However, trench IGBTs include atrench within which the gate is situated. The trench reduces theon-state voltage drop of the device. The current path of planar andtrench IGBTs is vertical from the collector to the emitter.

Lateral IGBTs (LIGBT) are often employed in lower power control anddetection circuits. Lateral IGBTs do not utilize the vertical structureof the planar and trench IGBTs, where collector and emitter contacts areprovided on the top and on the bottom of the semiconductor material.Instead, lateral IGBTs generally include a substrate contact at a bottomside, a collector at one side of a top side, an emitter at the otherside of the top side, and a gate disposed between the emitter and thecollector at the top side. The current path of lateral IGBTs ishorizontal (e.g., lateral) within the device from the collector to theemitter.

BRIEF DESCRIPTION OF THE DRAWINGS

Particular embodiments in accordance with the disclosure will now bedescribed, by way of example only, and with reference to theaccompanying drawings:

FIG. 1 is a schematic of an IGBT;

FIGS. 2-5 are top views and cross sections of embodiments of an LIGBTwith a n+ injection region in the anode;

FIG. 6 is a plot illustrating improvements in saturation current flowthrough the LIGBT of FIG. 5;

FIGS. 7-9 illustrate a schematic, a top view, and a cross sectional viewof another embodiment of an LIGBT with a p+ injection region;

FIG. 10 is a cross sectional view of another embodiment of an LIGBT;

FIG. 11 is a flow chart illustrating a method for fabricating an LIGBT;and

FIG. 12 is a block diagram of an integrated circuit that includes anLIGBT.

Other features of the present embodiments will be apparent from theaccompanying drawings and from the detailed description that follows.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE DISCLOSURE

Specific embodiments of the disclosure will now be described in detailwith reference to the accompanying figures. Like elements in the variousfigures are denoted by like reference numerals for consistency. In thefollowing detailed description of embodiments of the disclosure,numerous specific details are set forth in order to provide a morethorough understanding of the disclosure. However, it will be apparentto one of ordinary skill in the art that the disclosure may be practicedwithout these specific details. In other instances, well-known featureshave not been described in detail to avoid unnecessarily complicatingthe description.

Drain extended MOS devices may show compression in the outputcharacteristics at low drain currents due to high injection effects inthe drain extension regions. Drain extended devices may have limitedsaturated drain current (IDSAT) due to the low doping in the drainextension. The drain extension region also affects die size for largecurrent drivers. An IGBT device may enhance the ON-current of the drainextended devices by injecting minority carriers into the drain extensionregion by introducing a p+ region into the drain extension. Conductivitymodulation may be obtained by effectively using the p+ region in thedrain extension as the drain. Experiments have demonstrated a 3×improvement in IDSAT using this configuration.

A lateral IGBT construction is disclosed herein that allows forindependently improving the inherent diode for performance optimization.Another embodiment allows for adjusting the injection levels and PNPgain for improving safe operating area (SOA). In another embodiment,robustness against SCR (silicon controlled rectifier) action may beprovided by controlling the amount of minority carrier injection intothe drain extension region. In effect, the gain of the pnp portion ofthe IGBT may be reduced in order to inhibit SCR action and allow safeoperation over a wider range of voltage and current.

The solution is highly area efficient and widely applicable. Embodimentsof this disclosure may provide smaller devices, particularly withregards to electro-static discharge (ESD) circuits.

FIG. 1 is a schematic of a well known IGBT 100. The general operation ofan IGBT is well known and need not be described in detail herein, see,for example: “Novel Power Devices for Smart Power Applications,” JongMun Park, 2004. A brief description will be included herein in order toexplain the improvements disclosed herein. A major limitation of alaterally diffused MOSFET (LDMOSFET) is their relatively high specificon-resistance (Rsp) due to the majority carrier conduction mechanism.The IGBT is a relatively new power device which is designed to overcomethe high on-state loss of power MOSFETs. The device is essentially acombination of a pnp bipolar structure 101 which provides high currenthandling capability, and an n-channel MOSFET 102 which gives ahigh-impedance voltage control over the bipolar base current. It can befabricated as either a high-power discrete vertical IGBT or a low-powerlateral IGBT (LIGBT). LIGBTs may be integrated together with low voltagecontrol circuitry on a single integrated circuit.

The structure of LIGBT 100 is similar to that of an LDMOSFET in that thegate is also formed by double diffusion. The main difference between anLIGBT structure and an LDMOSFET is that the LIGBT has a p+ anode 103instead of the n+ drain of an LDMOSFET. In this structure, current flowcannot occur when a negative voltage is applied to the anode withrespect to the cathode 105, because the emitter junction J1 becomesreverse biased. Emitter junction J1 is the junction between emitteranode (E) and the n-buffer base region (B). This provides the devicewith its reverse blocking state, and the depletion region extends in then-drift region. When a positive voltage is applied to the anode 103 withthe gate shorted to the cathode, a collector junction J2 (n-drift andp-well junction) becomes reverse biased and the device operates in itsforward blocking state. Generally, the substrate is electricallyconnected to the source contact. When a positive gate voltage above thethreshold voltage is applied to gate 104 with respect to the cathode(source) 105, an inversion channel is formed that connects the n+cathode 105 to the n drift region. This creates the base current of thelateral pnp structure 101 in the LIGBT structure. If a positive voltageis applied between the p+ anode 103 and cathode 105, most of the voltagedrops across J1, until the junction becomes forward biased. Underforward bias any additional increase in voltage drops across the channeland the drift region. The holes from the anode are injected into the ndrift region and electrons flow into the drift region from the sourcethrough the channel. Because of the injected electrons and holes, the ndrift region becomes conductivity modulated. With further increase inthe anode voltage, more voltage drops across the inversion channel andthe electron current increases to compensate for the increased holecurrent. If the hole concentration exceeds the background doping levelof the n drift region, the device characteristics are similar to thoseof a forward biased pin diode. As a result, it can be operated at ahigher current density compared to conventional LDMOSFETs.

If the inversion layer conductivity is reduced by the gate bias close tothe threshold voltage, a significant voltage drop occurs across thechannel. When this voltage drop becomes comparable to the differencebetween the gate bias and the threshold voltage, the channel ispinched-off. At this point, the electron current saturates. As a result,the device operates with current saturation in its active region with agate controlled output current. In order to switch off the device, it isnecessary to discharge the gate by shorting it to the cathode.

When the gate to cathode voltage is reduced to zero, the device isswitched from its on-state to off-state; the current will fall to afraction of the steady state value due to the cut-off of the electroncurrent.

LIGBTs are susceptible to latch-up in the same way as discrete IGBTsbecause of an inherent parasitic pnpn thyristor in the device. At highcurrent levels, the voltage drop across the cathode-body junction issufficient to turn the parasitic npn structure on. The collector currentof the npn structure forms the base current for the lateral pnpstructure. When the sum of the current gains of the two structuresreaches unity, latch-up occurs and gate control is lost. A known methodto suppress latch-up is aimed at lowering the gain of the npn structureby using a p+ buried layer and a deep p+ sinker together with the nbuffer layer at the cathode region. This n buffer can help to reducecharge injection by controlling the emitter efficiency of the device.

FIG. 2 is a schematic of an embodiment of an LIGBT 200 with an n+injection region in the anode. LIGBT 200 is similar to LIGBT 100 ofFIG. 1. LIGBT 200 is essentially a combination of a pnp bipolarstructure 201 which provides high current handling capability, and ann-channel MOSFET 202 which gives a high-impedance voltage control overthe bipolar base current. Current flow cannot occur when a negativevoltage is applied to the anode 203 with respect to the cathode 205,because the emitter junction J1 becomes reverse biased. Emitter junctionJ1 is the junction between emitter anode (E) and the n-buffer baseregion (B). A control voltage applied to gate 204 controls current flowthrough LIGBT 200 in a similar manner as described with reference toFIG. 1 for LIGBT 100.

A problem with LIGBTs is that current flow may be limited by the voltagedrop across intrinsic diode J1. In this case, it may be desirable toincrease the size of the J1 diode, but this would in turn increase thesize of the entire LIGBT 200. In this embodiment, an n+ region is addedto the drain region to allow a contact point 212 for an additionalexternal diode 211 that may be added essentially in parallel withintrinsic diode J1. The other end of diode 211 may be coupled to a pad210 that is also coupled to anode 203. Pad 210 may then be coupled to asupply voltage, for example.

Diode 211 may be sized to handle a portion of a load current, in whichcase a portion 11 flows through the anode 203 and intrinsic junction J1and a portion 12 flows through external diode 211. Total voltage dropacross the two parallel diodes is less because current density is lower.

FIG. 3 is a cross sectional view of LIGBT 200. Semiconductor substrate320 may be derived from a silicon wafer, for example. The growing andprocessing of silicon wafers is well known, so only a brief summary willbe provided herein. An epitaxial (“epi”) layer 321 may be grown acrossthe top surface of substrate 321. Epi layer 321 and substrate 320 aretypically doped to be p-type, with a dopant concentration typically in arange of 1E16-5E17/cm3. A buried n-type layer 322 may be implanted inepi 321. A deep n-type well (dnwell) 323 may be formed in epi 321 bydiffusion or implantation, for example. Dnwell 323 forms the drainextension region for LIGBT 200. Dnwell 323 is a part of the drift regionconsistent with the above description. Alternatively, the epi layer 321may be doped n-type and function as the drain extension for LIGBT 200. Aheavier doped shallow nwell (SNW) 331 may be formed in dnwell 323 toform the drain region. SNW 331 may also be a part of the drift regionconsistent with the above description. P+ region 334 is formed in SNW331 and it may serve as the anode 203, referring back to FIG. 2. P+region 334 may be doped at a concentration of greater than 1E19/cm3, forexample. SNW 331 may be doped at a concentration of approximately3-5E17/cm3, for example. Deep n-type well 323 may be doped at aconcentration of approximately 1-2E16/cm3, for example. Double diffusedwell (dwell) 333 is a p-type diffusion and may be doped at aconcentration of approximately 1E18, for example. N-type region 335 isformed within dwell 333 and may serve as the source region for the MOSstructure 202, referring back to FIG. 2. P+ type contact region 337 isformed in n-type region 335 and in contact with p-type dwell 333 and itmay serve as the cathode 205, referring back to FIG. 2. P+ region 334may also serve as an emitter, whereas the n-type region 331 and n-typeextension region 323 may act as a base, and whereas p-type dwell 333 mayserve as a collector to form pnp structure 201, referring back to FIG.2. P+ region 334 and n-type region 331 form intrinsic diode junction J1,referring back to FIG. 1. A channel region 324 lies between drainextension region 323 and source region 335 and is covered by a thin gateoxide. Polysilicon gate structure 325 is formed over channel region 324to form MOSFET 202, referring back to FIG. 2.

In this example, LIGBT 200 is a double sided device, in that there is asecond SNW drain 332 with p+ anode 339 formed in dnwell 323 as anotherparallel drain. Polysilicon gate structure 326 may be a mirror image ofgate 325.

In this embodiment, an n+ contact 336, 338 is provided in drain/baseregions 331, 332 to allow an external diode 211 to be coupled to pad 210in parallel with the intrinsic emitter junction J1, as discussed abovewith regard to FIG. 2. N+ contact 336, 338 may be doped at aconcentration of greater than 1E19/cm³, for example.

FIG. 4 is a top view of an example multi-finger LIGBT transistor 200. Inthis example, metal interconnect layers have been removed in order tomore clearly see an aspect of the underlying structure. A first fingerof transistor 200 includes a source region stripe 335 and asubstantially parallel drain region stripe 331 with p+ anode region 334(see FIG. 3) that lie within semiconductor substrate material 320. Inthis example, a second substantially parallel drain region stripe 332with p+ anode region 339 (see FIG. 3) shares source region stripe 335 Insome embodiments, there may be only one finger that may include only onesource region stripe 335 and one drain region stripe 331, for example.In other embodiments, there may be only one finger that may include onlyone source region stripe 335 and two drain region stripes 331, 332, forexample. In yet other embodiments, there may be multiple fingers inwhich additional substantially parallel source region stripes 435 anddrain region stripes 439 are included. In the case of multiple sourceand drain region stripes, conductive interconnects may be used toconnect the drain region stripes 331, 332, 439, etc in parallel and toconnect the source region stripes 335, 435, etc in parallel to form asingle transistor with multiple parallel fingers. The conductiveinterconnects may be metallic, for example. In other embodiments, theconductive interconnects may be polysilicon, silicide, or other known orlater developed conductive interconnect materials. Semiconductorsubstrate material 320 is typically silicon; however other embodimentsof the disclosure may be applied to other semiconductor materials, suchas germanium, etc.

A channel region stripe is located substantially parallel to and betweeneach of the source region stripes and the drain region stripes. Eachchannel region has a width 401, and the total effective channel width ofdevice 200 is the sum of the widths of all of the channel regions of allof the fingers.

As described above with regard to FIG. 2, a set of n+ contacts 336, 338may be provided in each drain region 331, 332, 432 to allow coupling toan external diode, such as diode 211 of FIG. 2.

FIG. 5 is a schematic of another embodiment in which LIGBT 200 has aresistor 540 coupled in series with anode 203. Resistor 540 may allow areduction in SCR latching action and therefore allow safe operation overa wider range of voltage and current. As mentioned with regard to FIG.1, one way to reduce SCR action is to provide heavy p-type doping in thesource region dwell. In this embodiment, resistor 540 allows a reductionin the current that flows through anode 203 and that contributes to SCRaction. This current reduction may be compensated by an extra currentthat is allowed to flow through external diode 211 that does notcontribute to SCR action. Resistor 540 may be implemented using knownintegrated circuit fabrication techniques, such as an element using abulk resistance of a doped semiconductor region, a polysilicon element,an MOS device biased in the ohmic region, a metallic element, etc.

FIG. 6 is a plot illustrating improvements in current flow through LIGBT200 of FIG. 5. In this example, the term “anode current” includes boththe current that flows through anode contact 203 and the current thatflows through second diode 211. Plot lines 601 illustrate anode currentI_(anode) for gate to source voltages of 5v and 6v, but with no resistor540. Plot line 602 illustrates anode current Idrain for gate to sourcevoltages of 6v with a 50 ohm resistor 540. Plot lines 603 illustrateanode current Idrain for gate to source voltages of 6v and 7v with a 10ohm resistor 540. Plot lines 603 illustrate approximately a 30% increasein anode saturation current over plot lines 601.

FIG. 7 is a schematic of an embodiment of an LIGBT 700 with a p+injection region 712 in a deep n-type ring that surrounds LIGBT 700. Inthis case, second diode 711 may be included as part of LIGBT 700. Thisapproach may require less space than using a separate diode; however,this embodiment may not be amenable to using a resistor in series withthe anode to reduce SCR action, as described above in FIG. 5.

FIG. 8 is a cross sectional view of LIGBT 700. In this embodiment, then+ contact regions may be removed from drift/base region 331, 332. Aheavily doped deep n-type ring 840 may be implemented that goes down andjoins buried n layer 321. Deep n ring 840 may surround LIGBT 700, forexample. A p+ contact 836 is then formed in deep n ring 840 and may becoupled to pad 210, for example. In this manner, second diode 711 isformed by the junction of p+ 836 and heavily doped deep n ring 840.

In this case, current 841 may be injected into deep n ring 840, flowdown to buried n layer 321, then flow up into drain/base region 331 andthen to source/cathode 335.

FIG. 9 is a top view of LIGBT 700 illustrating deep n ring 840 with p+contact 836 which surrounds the multi-finger LIGBT device.

FIG. 10 is a cross sectional view of another embodiment of an LIGBT1000. As mentioned earlier with regard to FIG. 5, added resistance 540does not work for controlling SCR action with the integrated approach inwhich the second diode is formed in the deep n ring, as described withregard to FIG. 7. The drain extension region 323 is by necessity lowdoped to support voltage; therefore the pnp portion of an LIGBT with anintegrated second diode may have a high gain and cause more scrformation.

In the embodiment of FIG. 10, the p+ anode is removed from the drainextension and placed in a heavily doped deep n-type region 1031, 1032that goes down and joins deep n layer 321. The effective gain of theintrinsic pnp structure may be controlled by how much the deep n regionoverlaps the p+ region 1034, 1039, as indicated by arrow 1042. If itoverlaps just a little bit, then there is only a small base region thatresults in a higher gain pnp structure. However, if the deep n region1031, 1032 overlaps the p+ regions 1034, 1039 by a lot, then theeffective gain of the intrinsic pnp structure is reduced and SCR actionis thereby reduced and SOA is improved. For example, since the deepn-type region 1031 diffuses a lot, coincidence with p+1034 by using asame mask for both diffusions may provide a sufficient overlap. In an0.18 um process, a 2 um overlap may be effective to prevent SCR action.

In this case, all of the anode current 1041 goes down to the nbl 321,and then gets collected by cathode 1037 in dwell 333. This can be donewithout reducing performance of LIGBT 1000 because the benefit ofmodulating the conductance of the drain extension region under controlof the gate is still realized, which is what is limiting the current,but at the same time the tendency to turn on SCR action is reduced.

FIG. 11 is a flow chart illustrating a method for forming a second diodein an LIGBT transistor, such as LIGBT transistor 700 of FIGS. 7-9. Asmentioned above, the general operation of LIGBT transistors is wellknown. Similarly, the semiconductor process for fabricating an LIGBTtransistor is well known. Therefore, only the key fabrication stepsbased on this disclosure will be described in detail herein.

Initially, a semiconductor wafer is processed to form an epitaxial layeron top of the semiconductor wafer in step 1101. Nwell, pwell, and dwellregions are then patterned and diffused into the epi layer, asillustrated in FIG. 8, using known or later developed fabricationtechniques.

A mask is then applied in step 1102 to form field oxide regions. Anoxidation step is then performed to grow the thick field oxide in thedrift regions and source regions as illustrated in FIG. 8. A thin gateoxide layer may then be grown over the wafer.

A p+ region, such as 836 in FIG. 8, may then be formed in step 1103 in adeep n well region such as 840, which surrounds the LIGBT device to forma second diode junction such as diode 711 of FIG. 7.

Additional diffusions may be performed in step 1104 to form the p+ andn+ drain and source region stripes described in more detail above withregards to FIGS. 8, 9.

Additional fabrication steps may then be performed in step 1105 todeposit a polysilicon layer and etch it to form the polysilicon gatestructures, followed by one or more insulative layers and conductivelayers that are patterned and etched to form interconnects, etc. Viasand contacts may be formed between the metal interconnects and thepolysilicon gate structures.

After the semiconductor processing is completed, wafer testing isperformed in step 1106, followed by a sawing operation to separate thedie, packaging, and final testing of the integrated circuit.

SYSTEM EXAMPLE

FIG. 12 is a block diagram of an example system with integrated circuit(IC) 1200 that includes an LIGBT device. In this example, two LIGBTdevices 1201, 1202 are included, each of which may be similar to theLIGBT devices described with regard to FIGS. 1-10.

Control logic 1203 may also be included within IC 1200. Control logicmay be tailored to perform a particular control task, or may beimplemented as a processor core that may include memory for holdingsoftware and firmware instructions that may be executed by the processorto control the operation of LIGBT device 1201, 1202, for example.Additional interface logic, etc may be included within IC 1200.

Various types of systems may be implemented by connecting a load such asload device 1210 to be powered under control of IC 1200. Systems such asmicrowave and radio frequency (RF) power amplifiers may be implementedfor example. Various types of industrial, residential, commercial,medical, etc. systems may be implemented using power transistors thatare fabricated using the techniques disclosed herein to control motors,actuators, lights, etc.

OTHER EMBODIMENTS

While the disclosure has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various other embodiments of the disclosure will beapparent to persons skilled in the art upon reference to thisdescription. For example, while a LIGBT device was described herein,other embodiments may include other commonly known or later developedpower transistors, such as planar IGBTs, trench IGBTs, discrete IGBTs,etc.

While a multi-finger power transistor was described herein, otherembodiments may include a single finger power transistor. In someembodiments, there may only be a single drain stripe and a single sourcestripe.

While a linear transistor finger was described herein, in someembodiments, the finger topology may be other shapes than linear. Forexample, each finger may be configured as a circle, a square, arectangle, u-shaped, etc.

Certain terms are used throughout the description and the claims torefer to particular system components. As one skilled in the art willappreciate, components in digital systems may be referred to bydifferent names and/or may be combined in ways not shown herein withoutdeparting from the described functionality. This document does notintend to distinguish between components that differ in name but notfunction. In the following discussion and in the claims, the terms“including” and “comprising” are used in an open-ended fashion, and thusshould be interpreted to mean “including, but not limited to . . . .”Also, the term “couple” and derivatives thereof are intended to mean anindirect, direct, optical, and/or wireless electrical connection. Thus,if a first device couples to a second device, that connection may bethrough a direct electrical connection, through an indirect electricalconnection via other devices and connections, through an opticalelectrical connection, and/or through a wireless electrical connection.

Although method steps may be presented and described herein in asequential fashion, one or more of the steps shown and described may beomitted, repeated, performed concurrently, and/or performed in adifferent order than the order shown in the figures and/or describedherein. Accordingly, embodiments of the disclosure should not beconsidered limited to the specific ordering of steps shown in thefigures and/or described herein.

It is therefore contemplated that the appended claims will cover anysuch modifications of the embodiments as fall within the true scope andspirit of the disclosure.

What is claimed is:
 1. A transistor, comprising: a semiconductorsubstrate; a first p+ region formed within an n-type region adjacent ap-type dwell region that together form a pnp structure within thesemiconductor substrate, wherein an intrinsic diode is formed by the p+region and the n-type region; a second n-type region formed within thep-type dwell region; an insulated conductive gate lying above the p-typedwell region configured to control a channel region in the p-type dwellregion; a pad coupled to the first p+ region; and a second diode coupledbetween the pad and an n-type drift region in parallel with theintrinsic diode.
 2. The transistor of claim 1, further including an n+region formed in the n-type region, in which the second diode isconnected to the n+ region.
 3. The transistor of claim 1, in which thesecond diode is formed by a second p+ region formed within a deep n-typering that surrounds the power transistor formed in the semiconductorsubstrate.
 4. The transistor of claim 3, wherein the deep n-type ringoverlaps a portion of the first p+ region, such that gain of the pnpstructure is reduced.
 5. The transistor of claim 1 in which the seconddiode is a discrete diode device formed in the semiconductor substrate.6. The transistor of claim 1, further comprising a resistive devicecoupled in series with the interface pad and the first p+ region.
 7. Thetransistor of claim 1 having at least one finger, in which the at leastone finger has a linear topology.
 8. The transistor of claim 1 being aninsulated gate bipolar transistor (IGBT) and further including a thirdp+ region formed in the second n-type region; in which the first p+region serves as an anode of the IGBT; and in which the third p+ regionserves as a cathode of the IGBT.